The proposed research will investigate factors that may be important in determining the lineages of two precociously developing neurons in the frog embryo, Rohon-Beard (RB) neurons and primary motoneurons (PMN). Quantitative fate maps for these neurons, derived from intracellular injections of the lineage marker, horseradish peroxidase, have recently will constructed for the 16- and 32-cell stage blastulae. The proposed experiments will perturb the normal blastomere arrangement in a variety of ways in order to determine, by comparison to the normal quantitative fate map, whether the phenotype and number of these two specific neurons are determined to arise from particular blastomers, or to arise from any blastomere located in the appropriate position. Furthermore, the non-CNS descendants of these blastomeres will be mapped, and compared to normal maps. These experiments are important for the study of the effect of genomic, cytoplasmic, or cell-to-cell communication influences on the commitment of embryonic cells to the nervous system. Specifically, the neuronal and non-neuronal lineages of various identifiable blastomeres will be mapped after manipulations that affect gap junctional communication determination or commitment to lineage are transferred between cells during the early cleavage stages. Cytoplasmic transplantation and inhibition of metabolic by the 32-cell stage of development, in order to locate the subcellular moiety or process controlling this lineage determination. Finally, single cell transplantations and embryo- half separations will be performed to investigate the role of direct cell contact on those blastomeres that seem to be induced by their neighbors to contribute to CNS lineages. The results of these experiments will identify the cellular interactions that are necessary at cleavage stages for the expression of neuronal lineages, and will allow us to analyze what developmental decisions lead to the formation of the vertebrate nervous system.